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  Linux device mapper
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  <h2 id="1-功能："><a href="#1-功能：" class="headerlink" title="1.功能："></a>1.功能：</h2><p>Linux device mapper 可以将多个物理设备合成为一个逻辑设备，可以做普通的合并，或者实现类似<code>raid0</code>的条带化，还可以用来屏蔽硬盘中的坏道扇区，还可做lvm快照来备份数据库，或者通过零设备文件来模拟非常大的设备，用于测试功能。 <code>Device mapper</code> 是<code>lvm</code> 和 <code>multipating</code> 的底层技术。</p>
<h2 id="2-安装包："><a href="#2-安装包：" class="headerlink" title="2.安装包："></a>2.安装包：</h2><figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">device-mapper</span><br><span class="line">device-mapper-multipath</span><br></pre></td></tr></table></figure>

<h2 id="3-工作原理："><a href="#3-工作原理：" class="headerlink" title="3.工作原理："></a>3.工作原理：</h2><p>通过mapping table来创建逻辑设备（物理设备和逻辑设备每个扇区之间的对应关系）。</p>
<p>表内容包括：<br> 逻辑设备的起始扇区：<br> 通常是0 逻辑设备的扇区数量 类型（<code>linear</code>线性，连续组合；<code>striped</code>条带化；<code>error</code>屏蔽坏道；<code>snapshot</code>快照；<code>zero</code>零设备）</p>
<h2 id="4-创建逻辑设备映射表："><a href="#4-创建逻辑设备映射表：" class="headerlink" title="4.创建逻辑设备映射表："></a>4.创建逻辑设备映射表：</h2><p>1扇区&#x3D;512字节b</p>
<p>比如10G的磁盘，扇区数为：20000000个扇区.</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash">blockdev --getsize /dev/sda6 查看设备扇区数量</span></span><br><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">echo</span> <span class="string">&quot;0 `blockdev --getsize /dev/sda6` linear /dev/sda6 0&quot;</span> | dmsetup create mydevice</span> </span><br></pre></td></tr></table></figure>

<p>通过table创建逻辑设备 0表示这个逻辑设备从0扇区开始，有208782个扇区，linear表示连续的,&#x2F;dev&#x2F;sda6 0  表示从&#x2F;dev&#x2F;sda6的第0个扇区开始做逻辑设备。当一个设备用剩余空间做逻辑设备时候，扇区就不是从0开始了。 以下写入开机脚本后重启才会生效</p>
<h2 id="5-linear类型设备-特性："><a href="#5-linear类型设备-特性：" class="headerlink" title="5.linear类型设备 特性："></a>5.linear类型设备 特性：</h2><p>把多个物理分区的扇区连续起来组合成一个逻辑设备。</p>
<p> 0 20000 linear &#x2F;dev&#x2F;sda1 0 </p>
<p>20000 60000 linear &#x2F;dev&#x2F;sdb1 0</p>
<p> 注： 逻辑设备从0到扇区取自sda1从0扇区开始取20000扇区，逻辑设备从20000扇区开始，从sdb1的0扇区开始取60000个扇区，该逻辑设备80000个扇区。 实现命令</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">echo</span> “0 20000 linear /dev/sda1 0\n20000 60000 linear /dev/sdb1 0” | dmsetup create mydevice</span></span><br></pre></td></tr></table></figure>

<h2 id="6-stripe条带化："><a href="#6-stripe条带化：" class="headerlink" title="6.stripe条带化："></a>6.stripe条带化：</h2><p>通过<code>chunksize</code> 轮流写入磁盘 </p>
<p>0 1024 striped 2 256 &#x2F;dev&#x2F;sda1 0 &#x2F;dev&#x2F;sdb1 0 </p>
<p>注： 逻辑设备从0扇区开始到1024扇区，类型为striped ,2个设备, chunksize 256kb 从&#x2F;dev&#x2F;sda1和&#x2F;dev&#x2F;sdb1 的0 扇区开始 各取512扇区(注意扇区数必须是chunksize的倍数) 命令实现</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">echo</span> “0 1024 striped 2 256 /dev/sda1 0 /dev/sdb1 0” | dmsetup create mydevice</span></span><br></pre></td></tr></table></figure>

<h2 id="7-error"><a href="#7-error" class="headerlink" title="7.error:"></a>7.error:</h2><p>通过合成逻辑设备去除错误扇区 0 80 linear &#x2F;dev&#x2F;sda1 0 80 100 error 181 200 linear &#x2F;dev&#x2F;sdb1 0 命令实现</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">echo</span> “0 80 linear /dev/sda1 0\n80 100 error\n181 200 linear /dev/sdb1 0” | dmsetup create mydevice</span></span><br></pre></td></tr></table></figure>

<h2 id="8-snapshot逻辑卷快照-特性："><a href="#8-snapshot逻辑卷快照-特性：" class="headerlink" title="8.snapshot逻辑卷快照 特性："></a>8.snapshot逻辑卷快照 特性：</h2><p>创建快照后，出现3个设备（原设备，快照设备，cow设备） 如果数据没有改变，读取数据从原设备读取，写入变化的数据存储到cow区域中，快照设备保存原设备的数据。</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">echo</span> ―0 1000 snapshot /dev/sda1 /dev/vg0/realdev P 16 ‖ | dmsetup create mydevice从0扇区到1000扇区为/dev/sda1创建快照，名字为realdev,P表示下次启动仍然生效，16为chunksize</span></span><br></pre></td></tr></table></figure>

<h2 id="9-zero零设备特性："><a href="#9-zero零设备特性：" class="headerlink" title="9.zero零设备特性："></a>9.zero零设备特性：</h2><p>类似&#x2F;dev&#x2F;zero，但是他是个块设备，不能写东西，一般用来测试用，创建大的文件系统进行测试。比如测试创建10T大小的设备用ext3来格式化</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">export</span> HUGESIZE=$[100 * (2**40)/512] 100T的扇区数量 2**40为2的40次方</span></span><br><span class="line"><span class="meta prompt_">#</span><span class="language-bash"><span class="built_in">echo</span> <span class="string">&quot;0 <span class="variable">$HUGESIZE</span> zero&quot;</span> | dmsetup create zerodev 生成的文件在/dev/mapper/zerodev ext3每个分区最大支持2TB</span></span><br></pre></td></tr></table></figure>

<h2 id="10-多路径功能特性："><a href="#10-多路径功能特性：" class="headerlink" title="10.多路径功能特性："></a>10.多路径功能特性：</h2><p>多路径功能，用来提供线路冗余，监控每条链路，当链路失败时自动切换链路，而且自动恢复运行，防止单点故障。生成的设备名 &#x2F;dev&#x2F;dm-X 类型：<br> 当两路径优先级相等：<br> 负载均衡 当两路径优先级不等: 冗余</p>
<h2 id="11-multipath列出多路径设备"><a href="#11-multipath列出多路径设备" class="headerlink" title="11.multipath列出多路径设备:"></a>11.multipath列出多路径设备:</h2><p>后台需要开启multipathd服务，优先级大小为0-1024 实验步骤：</p>
<p>存储端配置双网卡，配置&#x2F;dev&#x2F;sda6为iscsi设备 服务器端安装device-mapper-multipath包，连接iscsi设备</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta prompt_">#</span><span class="language-bash">vi /etc/multipath.conf 注释掉 blacklist &#123; devnode <span class="string">&quot;*&quot;</span> 不同厂商的配置是不一样的 &#125; 取消注释 default&#123; udev_dir .. .. path_grouping_policy failover(根据失效域来判断执行策略) &#125;</span></span><br><span class="line"><span class="meta prompt_">#</span><span class="language-bash">systemctl <span class="built_in">enable</span> multipathd</span> </span><br><span class="line"><span class="meta prompt_">#</span><span class="language-bash">systemctl restart multipathd 之后生成的设备位置在/dev/mpath/下,可制作文件系统，挂载</span></span><br><span class="line"><span class="meta prompt_">#</span><span class="language-bash">multipath –ll 查询设备状态</span></span><br></pre></td></tr></table></figure>

<h2 id="12-FC存储"><a href="#12-FC存储" class="headerlink" title="12.FC存储:"></a>12.FC存储:</h2><p>存储端建立raid设备，raid建立与HBA卡WWN号的映射关系（连接哪个HBA卡则使用哪块磁盘设备） WWN为HBA卡的授权名称，用来区分一个或一组网络连接，表示网络上的一个连接。</p>
<h2 id="鸣谢："><a href="#鸣谢：" class="headerlink" title="鸣谢："></a>鸣谢：</h2><p>原文出自： <a target="_blank" rel="noopener" href="https://www.cnblogs.com/my-show-time/p/15152982.html">https://www.cnblogs.com/my-show-time/p/15152982.html</a></p>
 
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